In cellular telecommunication systems a single speech connection or data connection through the cellular telecommunication network is called a bearer. Generally, a bearer is associated with a set of parameters pertaining to data communication between a certain terminal equipment and a network element, such as a base station or an interworking unit (IWU) connecting the cellular network to another telecommunications network. The set of parameters associated with a bearer comprises typically for example data transmission speed, allowed delays, allowed bit error rate (BER), and the minimum and maximum values for these parameters. A bearer may further be a packet transmission bearer or a circuit switched bearer and support for example transparent or non-transparent connections. A bearer can be thought of as a data transmission path having the specified parameters connecting a certain mobile terminal and a certain network element for transmission of payload information. One bearer always connects only one mobile terminal to one network element. However, a bearer can pass through a number of network elements. One mobile communication means (ME, Mobile Equipment) may in some cellular telecommunication systems support one bearer only, in some other systems also more than one simultaneous bearers.
In order to be able to transmit information in a desired way, connections over the radio interface have to obtain a desired level of quality. The quality can be expressed for example as the C/I i.e. Carrier to Interference ratio, which indicates the ratio of received carrier wave power to received interfering power. Other measures for the quality of a connection are SIR i.e. Signal-to Interference ratio, S/N i.e. Signal to Noise ratio, and S(I+N) i.e. Signal to Noise plus Interference ratio. The bit error rate (BER) or frame error rate (FER) are also used as measures of connection quality. Typically, a certain target level for one of these or other corresponding measures is determined before hand, and for each connection, the transmission power is adjusted to be such that the target level is reached as closely as possible. The transmission power should not be higher than what is necessary for obtaining the desired target level, since a too high transmission level wastes electrical energy in the transmitting equipment, which is crucial with handheld mobile stations, and causes interference to other connections.
Admission control is a crucial function in ensuring, that each bearer obtains the desired SIR level. The purpose of admission control is to examine each new request for a new bearer, and determine whether the requested service can be provided without degrading the service to other bearers taking into account the transmission power of the requested bearer, the transmission bite rate of the bearer, the processing gain, and the bearer quality requirements. If the new bearer can be serviced without harming other bearers, the request is admitted. Otherwise it is scheduled i.e. put on a queue, renegotiated or modified, or rejected. Admission control typically co-operates with power control, whereby the transmission power of some of the other bearers may be adjusted in order to guarantee the SIR target level of the other bearers.
Various admission control algorithms have been proposed in the past. The article “SIR-Based Call Admission Control for DS-CDMA Cellular Systems” by Zhao Liu and Magda El Zarki, I2 Journal on selected areas in communications, vol. 12, no. 4, pp. 638–644, May 1994, describes an algorithm based on the concept of residual capacity. Residual capacity is defined as the additional number of initial calls a base station can accept. If the residual capacity is larger than zero, new calls are admitted. The residual capacity is determined from measured SIR levels and a threshold SIR level.
Another algorithms are described in the article “call Admission in Power Controlled CDMA Systems” by Ching Yao Huang and Roy D. Yates, in proceedings of I2 VTS 46th Vehicular Technology Conference, Apr. 28–May 1, 1996, Atlanta, USA, pp. 1665–1669. In this article, two simple algorithms are presented. In the first algorithm, a new call is blocked when that new call would cause ongoing calls to transmit at maximum power. In the second algorithm, a new call is blocked if the total received powers measured at the base station exceeds a predetermined threshold.
These algorithms function well, when the calls i.e. bearers are relatively similar in terms of resource usage, and any admission tresholds are set to a level where the admission of a bearer does not increase the load too near to the maximum capacity.
However, these algorithms do not function well, when the bearers have widely varying properties, i.e. when the network needs to handle both low bit rate bearers such as normal speech bearers, and high bit rate bearers such as high-capacity data bearers or live video bearers. Such a variety of services will be provided for example by the UMTS cellular telecommunication system presently under development. For example, in the conventional algorithm in which a new call is allowed if the total received power measured at the base station is under a predetermined treshold, a high bit rate bearer may increase the network load too near to the maximum capacity. This can be prevented by lowering the threshold so that any high rate bearers allowed close to the threshold still do not increase the total load too much, but in that case, the low bit rate speech bearers end up being refused even if the remaining capacity could accommodate them.
In this specification, the term could region is used to mean a region of the cellular telecommunication system, which is controlled by a single admission control entity or process, i.e., the region whose transmissions are taken into account when deciding about the admission of a new bearer. A control region may comprise for example a sector of a cell, a cell, or a plurality of cells such as a routing area or a whole ratio access network.
In the following, various other functions of a cellular network controlling the bearers are described, namely load control, power control, and handover control.
The main task of load control (LC) is to ensure, that the cellular system is operating at a point, where high capacity is achieved without excessive usage of power in the mobile stations, while obtaining a good connection quality. The definition of the load limit, up to which the system can be allowed to be loaded, is a critical task for radio resource management. Since an overload situation may considerably undermine the performance of the network, it is essential to control the load in order to avoid an overload situation. Some examples of the main functions of load control are network balancing, adjustment of power control parameters and handover parameters, and congestion control.
The aim of power control (PC) is to adjust the power levels of the mobile stations and the base stations in order to obtain the desired signal level at the receivers at either end, i.e. take care of the near-far problem. The power control also takes care of changing the power levels as a response to large changes in shadowing and for example as a response for fast changes in SIR level.
Handover control (HC) takes care of managing the change of connections of a mobile station, when the mobile station moves from one cell to another.
Typically, these functions are implemented in a network element as software programs executed by the processing unit of the network element. The means performing these functions are in this specification called entities, i.e. a HC entity takes care of handover control, a PC entity takes care of power control, a LC entity takes care of load control, and an AC entity takes care of admission control.